Conventional armrests are generally mounted on two opposite sides of a chair. It is desirable that the armrests are adjustable to be positioned to suit an individual user. An adjustable armrest is disclosed in U.S. Publication No. 2003/0030317 A1 and allows a user to adjust the horizontal position and orientation thereof. With a horizontally slidable seat and a linkage pivotally connected to the slidable seat, and with engagement between a protrusion which is mounted on the slidable seat and a selected one of cavities formed in the linkage, a rest surface of the armrest is retained at a desired position and orientation. However, since the position and orientation of the rest surface depend on the engagement of the protrusion with the selected cavity, the selections available to a user are limited, and the armrest cannot be freely adjusted for meeting the requirements of different users.